The present invention relates to a heat-sensitive adhesive sheet which can bond opposed information-recorded faces provided by one or two sheets of recording material together by application of heat to form a laminated information-recorded material which can be peeled apart when desired. The laminated information-recorded material thus formed can be utilized as, e.g., notices, cards and postcards in which information to be concealed from view is recorded. In particular, the invention is concerned with a heat-sensitive adhesive sheet and a laminated information-recorded material utilizing such a sheet, which can take the form of a mailable card and enables communication of more information than a conventional postcard at the same postage.
Hitherto, notices and direct mails concerning personal information, such as notices for payment, receipts and notices of expiration, have been mailed in the form of sealed letter from the Social Insurance Agency, banks, brokerage firms, post offices, electric power companies, and so on. However, the drawback to mailing a document by a sealed letter is its high price, compared with sending it as a postcard. Therefore, mailing large quantities of sealed letters obliges an immense expense on the sender part.
For the purpose of remove such a drawback, various postcards capable of safeguarding the secrecy of the contents, the so-called confidential postcards, have so far been proposed and put to practical use. Such postcards are made, e.g., by opposing textual or/and pictorial information-recorded faces of two sheets of recording material via a heat-sensitive adhesive sheet and subjecting them to pseudo adhesion by application of heat thereto. The amount of information communicated to a receiver by the thus made confidential postcard can be increased to the same level as that by a sealed letter. And the secrecy of the information recorded inside such a postcard can be kept till a receiver peels apart either sheet of recording material and reads the information recorded on the inside.
As a more specific example of those proposals, mention may be made of a lettercard having on one side of a paper support a heat-sensitive adhesive layer on which information can be recorded (Japanese Jikkai Hei 1-148371, wherein the term xe2x80x9cJikkaixe2x80x9d means an xe2x80x9cunexamined published utility model applicationxe2x80x9d). Such a lettercard has a postcard size when it is folded in two, and the opposed faces of the folded heat-sensitive layer are pressed under heating so as to adhere to each other in a condition that the adhesion strength between them secures peel-apart capability. However, this lettercard has a defect that the information recorded on one face is liable to be transferred onto the other face because the recorded face is the surface of the heat-sensitive layer to be peeled part. Further, it has a drawback of difficulty in controlling the adhesion strength of the heat-sensitive adhesive layer. So it frequently occurs that the lettercard curls by delamination to make it hard to read the contents.
As another example has been proposed a heat-sensitive adhesive sheet enabling pseudo adhesion between information-recorded faces of two sheets of recording material and an information-recorded sheet material formed by interposing such an adhesive sheet between information-recorded faces of two sheets of postcard-size recording material and subjecting them to pseudo adhesion by application of heat thereto (Japanese Tokkai Hei 4-126298, wherein the term xe2x80x9cTokkaixe2x80x9d as used means an xe2x80x9cunexamined published patent applicationxe2x80x9d). Such a heat-adhesive sheet has a four-layer structure that a synthetic resin coating (as heat-sensitive layer), a support, a thermoplastic resin coating (as release layer) and a synthetic resin coating (as heat-sensitive layer) are arranged in the order of mention. So the information-recorded sheet material using the foregoing heat-adhesive sheet can be peeled apart at the interface between the support and the thermoplastic resin layer. However, such an information-recorded sheet material has a defect that, when heat is applied to two information-recorded sheets between which the adhesive sheet is interposed, the resin coatings of the adhesive sheet melt and extrude from the cut end thereof; as a result, the adhesive sheet is bordered with those resins to make it hard to open the seal.
Such a defect is remedied by the confidential postcard disclosed in Japanese Tokkai Hei 7-17166. This postcard utilizes a heat-sensitive adhesive sheet having the same four-layer structure as mentioned above. Therein, the thermoplastic resin layer provided on one side of a support has a specified Vicat softening temperature so as to ensure easy peel-apart capability but prevent spontaneous delamination. And the heat-sensitive adhesive layers provided on the other side of the support and on the thermoplastic resin layer respectively have a specified glass transition or Vicat softening temperature. Further, the 180xc2x0 peeling resistance at the interface between the support and the thermoplastic resin layer is adjusted to a specified range.
Under recent circumstances that various kinds of paper sheets, particularly paper sheets having weak surface strength, have been used in making confidential postcards for sending bills, detailed statements, direct mails and so on, it has turned out that even the foregoing heat-sensitive adhesive sheet gave rise to defects at the time of opening the seal. This is because the adhesive sheet causes slight extrusion of resin coatings from the cut end at the time of adhesion, or low peeling resistance cannot consistently be ensured therein.
Therefore, a first object of the invention is to provide a double-sided heat-sensitive adhesive sheet for forming an information-recorded sheet material, especially useful for a confidential postcard, which can be opened without damaging the information recorded on the inside even when recording materials having weak surface strength are employed.
A second object of the invention is to provide an information-recorded sheet material which causes no defects when the seal is open although it uses recording sheets having weak surface strength.
The aforementioned objects are attained with a double-sided heat-sensitive adhesive sheet, which comprises a support provided with a release layer on one side thereof and heat-sensitive adhesive layers which are formed on both sides of the support and have sufficient adhesion to the support but can be easily peeled apart from the release layer, wherein the release layer and the heat-sensitive adhesive layer has at their interface a 180xc2x0 peeling resistance of 10-30 g/50 mm in a peel test performed at a peeling speed of 500 mm/min. in an atmosphere of 20xc2x0 C. and 65% RH, and an information-recorded sheet material formed by interposing the foregoing heat-sensitive adhesive sheet between each opposed pair of information-recorded faces of recording material having at least two information-recorded faces on one side and integrating them into a laminate by application of heat thereto.
In other words, confidential postcards prepared using the present double-sided heat-sensitive adhesive sheets have a special advantage in that they are significantly improved in seal opening rate, regardless of nature of paper supports used therein.